1. Field of the Invention
The present invention relates to a technique to manufacture, without using mercury, lead and indium, zinc-alkaline batteries which use zinc as a negative electrode active material, an aqueous alkali solution as an electrolyte and manganese dioxide, silver oxide, oxygen and the like as a positive electrode active material and provides a method of manufacturing zinc-alkaline batteries which do not cause environmental pollution and are excellent in storage stability and dischargeability.
2. Description of Related Art
Since about 10 years ago, environmental pollution due to mercury of waste batteries has become a serious problem and there have been conducted researches for reducing the amount of mercury used in alkaline dry batteries. As a result, corrosion-resistant zinc alloys and inhibitors have been developed and at present, the amount of mercury contained in alkaline dry batteries has been able to be reduced to 250 ppm based on the weight of the battery and furthermore, mercury-free alkaline dry batteries have also been put on the market.
An attempt for manufacturing alkaline dry batteries without mercury has been made since mercury-containing alkaline dry batteries were developed and a number of patent applications and reports were made on corrosion-resistant zinc alloys and inorganic and organic inhibitors. For example, elements such as indium, lead, cadmium and thallium are known as materials having a high hydrogen overvoltage and these elements are used as additive elements for corrosion-resistant zinc alloys and furthermore, compounds (salts, oxides, hydroxides) of these elements are used as inhibitors (Japanese Patent Kokai (Laid-Open) No.1-105466).
Batteries in which pure zinc free from mercury is used as a negative electrode active material suffer from the problems that a vigorous corrosion reaction of zinc takes place with the generation of hydrogen and internal pressure of the batteries increases to expel the electrolyte and to deteriorate the resistance to leakage of electrolyte. Further, in the case of partially discharged batteries, the hydrogen generation rate at the zinc negative electrode increases and the resistance to leakage of electrolyte is further deteriorated. These problems are caused by the fact that mercury which inhibits the corrosion reaction by enhancing the hydrogen overvoltage at the surface of zinc has been exhausted. As mentioned above, for manufacturing alkaline dry batteries without mercury, indium, lead, cadmium, thallium and the like are used as additive elements for corrosion-resistant zinc alloys and compounds of these elements are used as inhibitors.
Among these indium, lead, cadmium and thallium, lead, cadmium and thallium are pollution materials like mercury and it is not desired to use these elements as additive elements even in a slight amount for preventing environmental pollution caused by batteries. Since indium is generally not regarded as harmful material and is high in corrosion preventing ability, it is known as an additive to a negative electrode of not only primary batteries, but also secondary batteries. In fact, indium is used in formulation of mercury-free alkaline batteries not only as an additive element for alloys, but also as an inorganic inhibitor to be added to alkaline electrolytes. However, chronic poisoning of indium has not yet been elucidated and the U.S. Academic Council for Industrial Hygiene (ACGIH) has prescribed the permissible concentration more severely than for lead.
The object of the present invention is to provide non-pollution alkaline dry cells by inhibiting corrosion of zinc without using indium, lead, cadmium and thallium as well as mercury.